In recent years, attention has been paid to fuel cells as electric sources for electric vehicles and stationary electric sources in concert with social requirements and movements on the background of energy and environmental problems. Fuel cells are classified into a variety of types according to kinds of electrolyte and kinds of electrode, in which typical examples include alkaline one, molten carbonate one, solid electrolyte one, solid polymer one. Of these, the spotlight of attention is focused on solid polymer electrolyte fuel cell which is able to be operated at low temperatures (usually not higher than 100° C.) and which is in recent years progressed in development and practical use as a low environmental pollution power source for automotive vehicle.
Configuration of solid polymer electrolyte fuel cell (PEFC) is in general a structure in which an electrolyte membrane-electrode assembly (MEA) is interposed between separators. MEA includes an electrolyte membrane interposed between a pair of electrodes, i.e., an anode and a cathode. Each electrode contains an electrode catalyst and an electrolyte exemplified by solid polymer electrolyte, and has a porous structure in order to diffuse reaction gas supplied from outside.
In solid polymer electrolyte fuel cell, it is possible to take out electricity to the outside through the following electrochemical reactions: First, hydrogen contained in fuel gas supplied to an anode (fuel electrode) side is oxidized to form proton and electron by catalytic particles as shown in a chemical formula (1) mentioned below. Subsequently, the produced proton reaches a cathode (oxygen electrode) side electrode catalyst layer through a solid polymer electrolyte contained in an anode side electrode catalyst layer and a solid polymer electrolyte membrane contacting to the anode side electrode catalyst. Additionally, electrons produced in the anode side electrode catalyst layer reach a cathode side electrode catalyst layer through an electrically conductive carrier constituting the anode side electrode catalyst layer, a gas diffusion layer contacting to the anode side electrode catalyst layer at a side opposite to the solid polymer electrolyte membrane, a separator and an outside circuit. Then, protons and electrons reaching the cathode side electrode catalyst layer react with oxygen contained in oxidizer gas to produce water as shown by chemical formula (2) mentioned below.[Chem. 1]Anode reaction (fuel electrode): H2→2H++2−  (1)Cathode reaction (air electrode): 2H++2−+1/2O2→H2O  (2)
In an operation condition of low humidity and high current density, the amount of water moving with protons through the solid polymer electrolyte membrane from the anode to the cathode and the amount of produced water produced and aggregated in the cathode side electrode catalyst layer increase. At this time, this produced water stays in the cathode side electrode catalyst layer, inviting a flooding phenomena to occlude pores serving as reaction gas supply passages. By this, diffusion of the reaction gas is impeded to obstruct the electrochemical reaction thereby inviting lowering in cell performance.
Accordingly, hitherto a variety of trials to prevent the flooding phenomena by improving a water-drainage of gas diffusion layer. For example, in Patent Citation 1, a water retaining layer including a water retaining material, an electronically conductive material and a crystalline carbon fiber is disposed between a catalyst layer and a gas diffusion layer coated with a water repellent layer. It is disclosed that the existence of the crystalline carbon fiber can provide a solid polymer electrolyte fuel cell which ensures a water-drainage of the water retaining layer and has a stable electricity generating performance which is difficult to be affected by humidity fluctuation even upon fluctuation of relative humidity in gas.